Congestive heart failure (CHF) is a major public health issue in the developed and developing world. In the U.S., CHF affects more than 5.7 million people with 550,000 new cases diagnosed each year. Approximately 20% of hospitalizations are due to acute CHF, incurring a health-care system cost of $37.2 billion (AHA statistics, 2009). Heart failure has two main forms: systolic dysfunction and diastolic dysfunction. Some people with heart failure have both types of dysfunction. In systolic dysfunction, the heart contracts less forcefully and cannot pump out as much of the blood that is returned to it as it normally does. As a result, more blood remains in the lower chambers of the heart (ventricles). In diastolic dysfunction, the heart is stiff and does not relax normally after contracting, which impairs its ability to fill with blood. The heart contracts normally, but is unable to pump a normal proportion of blood out of the ventricles because filling was sub-optimal. Often, both forms of heart failure (systolic and diastolic) occur together. Although systolic heart failure is more commonly mentioned, there is growing recognition that congestive heart failure (CHF) caused by a predominant abnormality in diastolic function (i.e., diastolic heart failure) is both common and causes significant morbidity and mortality. Diastolic heart failure can occur alone or in combination with systolic heart failure. In patients with isolated diastolic heart failure, the only abnormality in the pressure-volume relationship occurs during diastole, when there are increased diastolic pressures with normal diastolic volumes. When diastolic pressure is markedly elevated, patients are symptomatic at rest or with minimal exertion (NYHA class III to IV). With treatment, diastolic volume and pressure can be reduced, and the patient becomes less symptomatic (NYHA class II), but the diastolic pressure-volume relationship remains abnormal.
In patients with systolic heart failure, there are abnormalities in the pressure-volume relationship during systole, which includes decreased ejection fraction (EF), stroke volume, and stroke work. In addition, there are changes in the diastolic portion of the pressure-volume relationship. These changes result in increased diastolic pressures in symptomatic patients, which indicate the presence of combined systolic and diastolic heart failure. Whereas the diastolic pressure-volume relationship may reflect a more compliant chamber, increased diastolic pressure and abnormal relaxation reflect the presence of abnormal diastolic function. Thus, all patients with systolic heart failure and elevated diastolic pressures likely have combined systolic and diastolic heart failure.
Another form of combined systolic and diastolic heart failure is also possible. Patients may have only a modest decrease in EF and a modest increase in end-diastolic volume but a marked increase in end-diastolic pressure and a diastolic pressure-volume relationship that reflects decreased chamber compliance. Therefore, all patients with symptomatic heart failure potentially have abnormalities in diastolic function; those with a normal EF have isolated diastolic heart failure, and those with a decreased EF have combined systolic and diastolic heart failure.
Heart failure typically begins after an “index event” produces an initial decline in pumping capacity of the heart. Following this initial decline in pumping capacity of the heart, a variety of compensatory mechanisms are activated, including the adrenergic nervous system, the renin angiotensin system and the cytokine system. In the short term these systems are able to restore cardiovascular function to a normal homeostatic range with the result that the patient remains asymptomatic. However, with time the sustained activation of these systems can lead to secondary end-organ damage within the ventricle, with worsening left ventricle (LV) remodeling and subsequent cardiac decompensation. As a result of resultant worsening LV remodeling and cardiac decompensation, patients undergo the transition from asymptomatic to symptomatic heart failure (Heart Failure Reviews, 10, 95-100, 2005).
In systolic heart failure, the LV undergoes a transformation from a prolate ellipse to a more spherical shape resulting in an increase in meridional wall stress of the LV, which in turn creates a number of de novo mechanical burdens for the failing heart. This LV remodeling dramatically alters the mechanical environment, which in turn influences growth and remodeling processes. A positive feedback loop emerges leading to acute dysfunctional cardiac pumping, pathologic neurohormonal activation, and the inability of the remodeled LV to respond appropriately to compensatory mechanisms.
Progressive LV dilation and subsequent remodeling is one of the mechanisms that lead to LV wall stress and myocardial stretch. Increased LV wall stress may lead to sustained expression of stretch-activated genes (angiotensin II, endothelin and tumor necrosis factor) and/or stretch activation of hypertrophic signaling pathways as stretch triggers myocyte responses both by inducing the release of humoral factors that are important in the initiation and maintenance of hypertrophy, as well as via the direct activation of signaling pathways as well.
LV dilation and increased LV sphericity are also sensitive indicators of poor long-term outcome. Thus, cardiac wall stress (which can be defined as the “force per unit of cross-sectional area”) of the ventricular wall is directly related to the difference in pressure between the ventricles and ventricular radius, and inversely related to ventricular wall thickness. So with LV remodeling, an increase in ventricular volumes and a subsequent increase in ventricular radius, a larger force is required from each individual myocyte to produce enough pressure in the ventricles. Wall tension is seen as a function of both internal pressure and vessel radius. Also, with ventricular remodeling, cardiac mass can increase, with a corresponding increase in ventricular wall thickness. Any such increase in wall thickness would result from remodeling at the cellular/extracellular matrix level by several processes including myocyte hypertrophy, cell slippage, and interstitial growth. However, such increases in wall thickness do not adequately compensate for the increase in wall stress resulting from cardiac chamber dilation with an increasing metabolic stress. Thus, ventricular remodeling is maladaptive, despite any incremental increase in ventricular wall thickness. Laplace's equation provides a framework for defining means of mitigating ventricular remodeling. Ventricular wall stress can be reduced by (1) decreasing transmural pressure, (2) reducing cardiac chamber radius, and/or (3) promoting greater ventricular wall thickness. A diastolic support device can have a significant impact on effective transmural pressure which can lead to a decrease in the diastolic wall stress and modulate the end-diastolic volume.
Of the 5.7 million people in the U.S. and 25 million people worldwide who suffer from heart failure, between 30-55% of these patients suffer from diastolic heart failure (DHF) and are without effective treatment. The term diastolic heart failure (DHF) generally refers to the clinical syndrome of heart failure associated with a preserved left ventricular EF, in the absence of major valvular disease. Forty percent of incident CHF cases and 50-60% of prevalent CHF cases occur in the setting of preserved systolic function. Mortality rate among patients with DHF is considered lower than in systolic heart failure. Some challenge this notion, showing that the natural history of patients with DHF may not be different from that of patients with systolic heart failure. The morbidity and rate of hospitalization are similar to those of patients with systolic heart failure. Due to its higher prevalence in the elderly population, the incidence of DHF is expected to rise with the increased aging of the western world population. The fundamental problem in diastolic heart failure is the inability of the left ventricle to accommodate blood volume during diastole at normal filling pressures.
Two basic types of diastolic abnormalities may be present, impaired ventricular relaxation, which primarily affects early diastole, and increased myocardial stiffness, which primarily affects late diastole. The rate and extent of the active relaxation may influence LV suction during the early filling phase. Both abnormalities lead to elevation of diastolic pressures. In DHF patients, a relatively small increase in central blood volume or an increase in venous tone, arterial stiffness, or both, can cause a substantial increase in left atrial and pulmonary venous pressures and may result in exercise intolerance and acute pulmonary edema. The mechanisms underlying abnormalities in diastolic function can be divided into factors intrinsic to the myocardium itself and factors that are extrinsic to the myocardium. Myocardial factors can additionally be divided into cellular and extracellular. Cellular factors include impaired calcium homeostasis leading to abnormalities in both active relaxation and passive stiffness, changes in sarcomeric proteins isotypes, such as titin, which acts as a viscoelastic spring that gains potential energy during systole and provides a recoiling force to restore the myocardium to its resting length during diastole. Since relaxation is an energy consuming process, any abnormalities in cellular energy supply and utilization can lead to impaired relaxation. Extracellular factors include changes in structures and quantity of the extracellular matrix, i.e. fibrosis, that lead to increased myocardial stiffness. There is limited data on neurohumoral markers in DHF patients other than natriuretic peptides (NPs). This probably reflects the fact that DHF has only recently been recognized as an important clinical problem. The present work is towards development of a novel diastolic recoil device to manage patients with diastolic heart failure.
For treating systolic heart failure there are several classes of solutions, e.g. pharmaceuticals, stem cells, electrical devices, mechanical devices, and surgical reconstruction. Each of these are designed for some limited target action (i.e., beta-blockade, ACE inhibition, electrical pacing, cardiac assist, etc); consequently, heart failure remains a cause of tremendous morbidity and healthcare burden. Conventional approaches fail to address the possibility that mechanical stimuli are important parameters for guiding growth and remodeling, processes that may ultimately facilitate the recovery of mechanical organs. The mechanical heart assist devices Class IIIA and IIIB are classified into active devices that provide pumping energy, and passive devices that modulate the shape of the heart. The active devices are subdivided into blood pumps, counter pulsation assist devices (aortic balloon pumps), and direct cardiac compression devices (DCCDs). The passive, “support” devices directly interact with the heart to change shape or limit growth.
Diastolic heart failure therapies presently include mostly pharmaceutical products and there are few, if any, devices available. There are presently no approved devices for treatment of the DHF symptoms. However, two preclinical stage recoil device concepts, LEVRAM and Imcardia have a potential role in the treatment of DHF patients. These and other devices are seen in U.S. Patent Application Publication No. 2008/0071134, In Vivo Device for Assisting and Improving Diastolic Ventricular Function; U.S. Patent Application Publication No. 2006/0276683, In-vivo Method and Device for Improving Diastolic Function of the Left Ventricle; and U.S. Patent Application Publication No. 2006/0241334, In vivo Device for Improving Diastolic Ventricular Function.
Cardiac strain patterns appear to be a major controller of cardiac stem cell differentiation into functional cardiomyocytes. The exact normal or physiologic strain pattern of the heart is not currently known. Tests to determine the normal strain pattern in the heart of eight healthy sheep using bi-plane x-ray data of radio-opaque markers produced eight distinctly different patterns. It appears that cardiac contraction is similar to gait; there are gross similarities amongst individuals (e.g., toe off and hip twist), but the details can be distinctly different (e.g., angle of leg at toe off, amount and timing of the hip twist). In fact, people can often be recognized from their gait. While it is difficult to describe a normal gait, it is quite easy to classify abnormal gaits. Likewise, normal cardiac strain pattern is difficult to define and prescribe, yet it is quite easy to identify abnormal cardiac strain patterns such as dyskinesis and hypokinesis.
It is well established that mechanical stimuli (e.g., stress or strain) are important epigenetic factors in cardiovascular development, adaptation, and disease. In the vasculature, for example, it appears that perturbed loading conditions heighten the turnover of cells (proliferation and apoptosis) and matrix (synthesis and degradation) in altered configurations, thus resulting in altered geometries, properties, and biologic function. Just as similar mechanisms appear to be operative in hypertension, aneurysms, and micro-gravity induced changes, it is likely that they are operative in cardiac disease.
Dyskinesis or aberrant motion of the myocardium during contraction is likely important in all diseases of the heart that involve remodeling of the myocardium. Clearly, borderzone myocardium is viable yet overloaded to the extent that it is dyskinetic, i.e., lengthens when it should shorten. It is likely that overloading leads to aberrant remodeling because offloading leads to: normalization of genes that regulate calcium handling, tumor necrosis factor and cytoskeleton proteins; regression of fibrosis and cellular hypertrophy, and improved in-vitro contractile function. Too much offloading is suspected to result in heart atrophy, whereby gradual weaning from a device should be sought along with combination therapy such as with clenbuterol.
At the cellular level, myofibrillar organization, sarcomere alignment and cell migration are all known to be mediated by mechanical factors. Mechanical factors are also known to play an important role in the behavior of stem cells, suggesting that understanding and control of the mechanical environment may be critical to the realization of the potential for stem cell therapies.
Cellular and subcellular investigations have established that altered hemodynamic loading leads to growth and remodeling of myocytes and extra-cellular matrix and myocytes are very sensitive to perturbations in strain and respond with altered gene expression. Abnormal cardiac kinematics is often considered as a symptom of heart failure when in actuality it may be a primary cause of the aberrant growth and remodeling. Other CHF mechanisms or co-contributors are, among others, loss of myocyte shortening capability, calcium dysregulation and unspecified myocyte apoptosis.
Regenerative therapies incorporating stem cells have demonstrated potential but have yet to be fully developed. Benefits observed in stem cell studies have been controversial, e.g. there is a general lack of evidence that implanted stem cells are actually integrating with the native tissue as functional cardiomyocytes. Stem cells are typically transplanted into the diseased myocardium where fiber alignment is highly disorganized and disrupted by fibrotic tissue. In the dyskinetic myocardium, the mechanical and environmental cues required to guide alignment and migration of transplanted cells are severely compromised. The device described herein, provides the means to restore motion that may be critical to establishing the appropriate physiologic mechanical environment required to optimize stem cell transplant therapies.
The various mechanical assist therapies (i.e., drugs, biventricular pacing, blood contacting assist devices, surgical manipulations, or passive stents and constraints etc.) typically off-load the heart and thus only modulate the strain pattern indirectly (e.g., through greater ejection fraction). Only direct cardiac compression devices (DCCDs) can directly induce a particular strain pattern. However, most prior DCCDs have been developed for enhancing ejection fraction or for ease of implantation rather than for strain modulation. Most induce aberrant strain patterns during contraction.
What follows is a discussion of the disadvantages of the prior art. FIGS. 1A-1D shows the normal, null, and inverted curvature in apex-to-base, radial plane (long axis) of the heart. FIGURE lA illustrates a normal or positive curve with the inside of the curve toward the chamber, where the top references the base and the bottom references the apex. FIG. 1B illustrates a null curvature. FIG. 1C illustrates an inverted or negative curvature where the inside of the curve is away from the chamber. FIG. 1d is an illustration that shows the curvature inversion of the Anstadt cup as illustrated in FIG. 9 of the Anstadt patent (U.S. Pat. No. 5,119,804). DCCDs have been characterized as most promising with good hemodynamics and ease of implantation. A number of DCCDs are being developed. The Anstadt cup is shown in FIG. 1D. The CardioSupport System by Cardio Technologies Inc. is similar to the Anstadt cup. The attachment is via vacuum on the apical end and the assist is via inflation of a membrane that lies between a rigid shell and the epicardial surfaces of the right ventricle (RV) and left ventricle (LV). The devices of Parravicini and the AbioBooster by Abiomed Inc. are sewn to the interventricular sulci, and elastic sacks between the shell and the epicardial surface are inflated during systole. The DCC Patch by Heart Assist Tech Pty Ltd is similar to the AbioBooster. It has been described as “ . . . two patches shaped to suit the profile of the heart . . . inflated and deflated in synchrony with the heart . . . ” The heart booster is composed of longitudinal tubes that have elliptical cross-sections with the major axis of the ellipse in the hoop direction.
To understand how all of these DCCDs induce aberrant strain patterns, it is important to note that contraction strain depends on both the end-diastolic configuration (reference configuration) and the end-systolic configuration (current configuration). The strain field is a function of the gradient (with respect to reference position) of the mapping of material points from the reference configuration to the current configuration. Thus, the fact that prior DCCDs fit the diastolic configuration is inconsequential to achieving an appropriate contraction strain pattern because their end-systolic configurations are grossly aberrant. Although strains induced by such motions as torsion may not perturb the heart geometry; if the overall geometry is abnormal, then the strain must be abnormal. Unphysiological geometries are illustrated in FIGS. 1A-1D.
Generally, the curvature is inversely proportional to the radius-of-curvature and that curvature changes sign when the origin of the radius-of-curvature changes sides. As should be evident from FIG. 1D, curvature inversion can greatly increase EF. However, the curvature of the ventricles in a normal heart does not invert during systole, thus rendering such motions grossly abnormal. A healthy heart, moreover, will resist having its curvature inverted and heart function needs to decline by 30% before the effect of “non-uniform direct cardiac compression” becomes noticeable. In short, the heart resists assist when a DCCD induces aberrant strains. DCCD devices described above induce motions that are grossly abnormal. The Vineberg device inverts curvature in long axis planes and short axis planes. The Anstadt cup and Cardio-Support System invert curvature in long axis planes yet preserve curvature in the short axis planes. The AbioBooster, DCC Patch, Hewson device, and Parravicini devices pull on the interventricular sulci and push on the freewall such that the curvature will increase at the sulci and decrease on the freewalls. The Heart Booster inverts curvature in short axis planes, yet preserves curvature in the long axis planes. Because they were not designed to eliminate aberrant motions, it should not be surprising that these existing DCCDs described above induce aberrant strain patterns.
Additionally, none of the existing DCCDs described above are implanted in a minimally invasive fashion, and such an implantation method is highly desirable, clinically useful, and commercially advantageous. Given that strain is a primary stimulus of myocardial growth and remodeling, there is a need for a DCCD that eliminates dyskinetic or hypokinetic motions in the heart.
This device, described in U.S. patent application Ser. No. 10/870,619, filed Jun. 17, 2004 (the '619 Application), which is incorporated by reference herein, is the first implantable device to proactively modulate the strain pattern during contraction. The class of devices claimed in the '619 Application are those that apply direct cardiac compression in a manner such that the end-diastolic and end-systolic configurations are physiologic with normal cardiac curvature, i.e. the class of direct cardiac compression device that achieve cardiac rekinesis therapy. The device disclosed in the '619 Application must be attached to the valve plane of the heart. An attachment developed in benchtop trials consists of suture runs along the right and left free walls together with stents that go from the device shell to the center of the valve plane via the transverse pericardial sinus (anterior stent) and oblique pericardial sinus (posterior stent). In addition to keeping the heart in the device, the stents eliminate the need to suture near the coronary arteries in the interventricular sulci. The highly elastic membrane on the epicardial surface is sealed tightly with the rigid shell to contain the pneumatic driving fluid (e.g., air). A typical membrane requires about 1 kPa (10 cm H20) of vacuum to unimpede heart filling. This is similar to that of the native heart which typically requires about 9 cm H20 of transmural pressure to fill (e.g., 6 cmH20 of venous pressure minus a negative 3 cm H20 of intrathoracic pressure). The pressure waveforms (with compression for systole and tension for diastole) were generated by a Superpump System made by Vivitro Systems Inc. for cardiovascular research. The sync out signal was amplified, made bipolar, and used to pace the heart via right atriam (RA) leads.
One method of overcoming some negative effects of a hard-shelled DCCD (e.g., the need for a large thoracotomy) is to use a soft-shelled device. Soft-shelled devices include DCCDs with primary components that are constructed out of highly deformable materials. Such DCCDs can be collapsed and possibly implanted through a small incision this is likely to be sub-xiphoid (e.g., inferior to the xiphoid process) or a left thoracotomy. The Abiobooster and Heart Booster are currently existing soft-shelled devices. However, as described above, both of these devices induce an aberrant strain pattern in the heart. Additionally, implantation methods for these devices still require sewing the devices to the heart or pericardium.
The above mentioned direct cardiac compression devices are active devices or assist devices that have a power source and method of delivering the power to increase cardiac output. Other devices that contact the outer surface of the heart are cardiac support devices and diastolic recoil devices. Cardiac support devices are useful for limiting the heart size, but they constrict the heart and thus impede filling (at best, they do not impede filling until some limit point where size of the heart is limited). Dynamically adjustable support devices are further useful because the limit point can be controlled to additionally decrease the size of an enlarged heart. Diastolic recoil devices are useful for increasing the recoil or filling of the heart, but they do not necessarily limit the heart size.
What is desired is a mechanical oriented device and therapy designed to optimize the mechanical environment for heart growth and remodeling that are restorative and potentially rehabilitative in nature.